Under the laws of thermodynamics, engines fueled by chemical reactions, e.g., reduction-oxidation (redox) reactions, require a substantial positive temperature differential between the inlet air, which must be cold and the exhaust gas, which must be hot. As a jet engine accelerates, especially above Mach 1, the temperature of the inlet air rises rapidly while the temperature of the exhaust gas rises more slowly, so the temperature differential diminishes. Eventually the temperature differential is extinguished and no positive work can be withdrawn from the engine, so it ceases to produce thrust. This happens at Mach 4 or so for a conventional turbine. One approach to higher speed attempts to avoid the inlet air temperature rise by compressing and decelerating the flow less and running the combustion process at supersonic speed. This approach is embodied in the supersonic combustion ram jet (scram jet). Compression is required to generate work, so the scram jet delays the onset of zero thrust to higher speed (Mach 8-10 range). The scram jet cannot overcome the problem of diminishing temperature differential and so, it too, is speed limited.
In chemical propulsion, the specific impulse is limited by the energy available when molecules combine. In contrast, with electric propulsion, energy is added from an external source. In principle, therefore, the specific impulse of electric propulsion can be as large as desired. In practice, the specific impulse is limited by the particular implementation. Since thrust will decrease as the specific impulse increases for a given power, a tradeoff must be made for a particular mission between propellant usage and mission time. High specific impulse leads to low propellant usage.
Plasma (also referred to as ionized gas) is an energetic state of matter in which some or all of the electrons have become separated from the atom. Excitation of a plasma requires then, at least partial ionization of neutral atoms and/or molecules of a medium. There are several ways to cause ionization including collisions of energetic particles, strong electric fields, and ionizing radiation. The energy for ionization may come from the heat of chemical or nuclear reactions of the medium, as in flames, for instance.
There are two broad categories of plasma, hot plasma and cold plasma. In a hot plasma, full ionization takes place, and the ions and the electrons are in thermal equilibrium. A cold plasma (also known as a weakly ionized plasma) is one where only a small fraction of the atoms in a gas are ionized, and the electrons reach a very high temperature, whereas the ions remain at the ambient temperature or slightly above. Cold plasma can be created by using a high electric field, or through electron bombardment from an electron gun, or by other means
There are three main types of electric thrusters: electrothermal, electromagnetic, and electrostatic. Electrothermal thrusters are similar to standard chemical rocket engines in that heat energy is added to a working fluid in a confined volume, raising its pressure, but differ in that the heat is produced by electrical means (often an electrical discharge). The gas is subsequently expanded through a converging-diverging nozzle to achieve thrust just as in chemical rockets. There are a variety of electromagnetic thruster configurations, but all depend on generating a thrust by accelerating particles in a direction perpendicular to both the electric and magnetic fields in the plasma. For example, the pulsed plasma microthruster (PPT) utilizes a spark discharge across a block of TEFLON® to create plasma, which is accelerated outward by induced azimuthal current interacting with a radial magnetic field. In a Hall thruster, an axial electric field provided in a radial magnetic field creates an azimuthal Hall current, which accelerates plasma axially producing thrust. In the self-field magnetoplasmadynamic (MPD) thruster, the current flow creates its own magnetic field in which the jxB force accelerates the plasma flow radially and axially. This can only occur if the current and hence the power are high, often necessitating pulsed operation at lower average powers. Electrostatic thrusters accelerate charged particles in an electric field, without an applied magnetic field. A linear accelerator, such as the one installed at the Stanford University Linear Accelerator Center (SLAC) is an example of an electrostatic thruster, though it is not used for the production of thrust.
In general, electromagnetic thrusters have much higher specific impulse than electrothermal thrusters. Electromagnetic thrusters are more compact than electrostatic ion thrusters because a charge neutral plasma does not have a space charge limitation on density. Problems include electrode erosion and general complexity of flow and current fields. The PPT thruster is mature and simple, but does not scale up to large powers.
Electrostatic ion thrusters use a set of grids to accelerate charged ions. Electrons are also expelled separately to maintain charge neutrality and prevent a charge buildup which could shut off the ion beam. Heavy gases such as mercury vapor and xenon have been used to reduce ionization losses as a fraction of total energy. Ionization losses are approximately the same for most gases, whereas for a given exhaust velocity the energy added per ion is greater for heavier gases.
In electrostatic thrusters, the beam consists of ions only and repulsion between particles limits the maximum density to relatively low levels, sometimes called the “space charge effect”. The space charge effect limits electrostatic thrusters to significantly lower thrust than other types of electric thrusters.
Consequently, there is no chemical-fuel based airbreathing propulsion system that can enable hypersonic flight in the range above Mach 10 or so and up to orbital speed. The range above Mach 10 is important because it would provide access to orbit without a rocket concept. The drawback of the rocket is largely size and, therefore, cost. A rocket capable of reaching the moon will typically have a fuel mass fraction of 90%+; of that fuel mass, 85% will be oxidizer. An airbreathing concept, as disclosed here in, would permit most of the oxidizer mass to be left off the vehicle.